Aircraft engine nacelles are prone to ice buildup. FIG. 1 shows a schematic representation of a typical high-speed jet engine assembly 1400. Air enters through inlet section 1414, between fan blade spinner 1416 and the annular housing 1418, which constitutes the forward most section of the engine nacelle 1420, and includes nacelle inlet lip 1421. Hot, high-pressure propulsion gases pass through the compressor section 1417 and the exhaust assembly (not shown) out the rear of the engine. An annular space or D-duct 1430 is defined by bulkhead 1428 and annular housing 1418. Bulkhead 1428 separates D-duct 1430 from the interior portion 1431 of the inner barrel 1412. In flight, under certain temperature and humidity conditions, ice may form on the nacelle inlet lip 1421, which is the leading edge of annular housing 1418, and on the fan blade spinner 1416. Accumulated ice can change the geometry of the inlet area between annular housing 1418 and fan blade spinner 1416, and can adversely affect the quantity and flow path of intake air. In addition, pieces of ice may periodically break free from the nacelle 1420 and enter the engine 1450, potentially damaging fan/rotor blades 1460 and other internal engine components.
Engine nacelles also channel fan noise from the engines, which can be a prime source of aircraft noise. As is known to those skilled in the art, aircraft engine fan noise can be suppressed at the engine nacelle inlet 1414 with a noise absorbing inner barrel liner 1440, which converts acoustic energy into heat. The liner 1440 normally consists of a porous face skin supported by an open cell backing to provide required separation between the porous face sheet and a solid back skin. This arrangement provides effective and widely accepted noise suppression characteristics. Aircraft engines with reduced noise signatures are mandated by government authorities, and as a result, are demanded by aircraft manufacturers, airlines and local communities.
The prior art includes designs for combating both noise and ice buildup on nacelle surfaces, and on nacelle inlets, in particular.
Others have developed an acoustically treated nacelle inlet having a hot air ice protection system. An acoustic liner positioned forward of the inlet throat has a perforated face skin, a perforated back skin, and an acoustic core between the face skin and the back skin. The openings through the face skin are sized to allow acoustic energy to be transmitted to and dissipated in the acoustic core, and the openings in the back skin are sized to channel hot gas from the engine through the acoustic liner to the surface of the inlet to heat the inlet and prevent and/or restrict ice formation on the inlet.
U.S. Published Patent Application No. 2005/006529, assigned to Rohr Inc., discloses an acoustically treated nacelle inlet having a low power electric heat ice protection system. As used herein, the term “low power” is intended to mean average electric power consumption less than about 1 watt per square inch (W/sq. in.). The electric power supply may be a conventional source such as batteries, or it may be the engine or an auxiliary power unit (APU), or a combination thereof.
FIG. 2 shows a schematic cross-sectional view of an inlet lip 1521 like that described in the above-identified published application. The bulkhead 1528 and the inlet lip 1521 define a D-duct 1530. The inlet lip 1521 includes a noise abatement structure, which in this embodiment is an acoustic panel 1504 comprising an open cell core 1508, a solid back skin 1509, and an acoustically permeable front skin 1510. The acoustic panel 1504 may be extended around the leading edge of the nacelle 1520 (as shown in dashed lines 1514 in FIG. 2), rather than ending at or near the leading edge 1505 of the nacelle 1520, as shown. A low power electric ice protection system (referred to herein by the acronym IPS) 1512 overlays the outer surface of the front skin 1510, in the manner described below.
FIG. 3 shows an exploded view of the acoustic panel 1504 shown in FIG. 2. The acoustic panel 1504 comprises a single degree-of-freedom open cell core 1508, a solid back skin 1509, and a perforated front skin 1510. Panels of this sort are well known to those skilled in the art. The perforations or openings in the front skin 1510 permit interaction between the open cell core 1508 and sound waves generated during operation of the gas turbine engine surrounded by the nacelle 1520. The open cell core 1508 is affixed via epoxy or other adhesive bonding to each of the skins 1510 and 1509. The sandwich structure defined by the core 1508, back skin 1509, and front skin 1510 can be made of either a metallic material, such as aluminum, a non-metallic material, such as a graphite/epoxy laminate, or a combination thereof.
The low power IPS 1512 is affixed using conventional bonding techniques (e.g., adhesive bonding) to the outer surface of the front skin 1510. The IPS 1512 is connected to an electric power supply or source (not shown in FIG. 2 or 3) by wiring. The IPS 1512 comprises an electrically conductive material that is permeable to sound waves, and can be a fine grid stainless steel wire mesh adhesively bonded to the outer surface of the perforated skin 1510. The fine grid wire mesh typically has a Rayl value between about 50-150, and preferably between about 70-110. The IPS is affixed to the skin 1510 of the acoustic panel 1504 in such a manner that it does not substantially block or otherwise interfere with a substantial number of openings in the skin 1510 of the acoustic panel 1504. This goal may be achieved by, e.g., selecting the size, shape and configuration of the wire mesh comprising the IPS 1512 vis-à-vis the size, shape and configuration of the perforation pattern in the skin 1510; and/or by using well-established bonding methodologies sufficient to minimize blocking the openings with wire mesh and the adhesive used to affix the mesh to the skin 1510. In prior art systems, typically no more than about 1-2% of the openings are completely blocked, although this figure may range as high as 5% or even 10%.
The prior art system of FIG. 2 also includes a parting strip heater 1507. The parting strip heater 1507 is adhesively bonded to the front skin 1510 at or near the highlight 1505 of the nacelle (and away from the IPS 1512), where the highlight 1505 is the peak of the curved nose of the nacelle. Parting strip heater 1507 comprises an electrifiable grid material preferably made of heavier gage wire elements as compared to the IPS 1512 wire mesh, in order to conduct a higher power electrical current.
FIG. 4 depicts a perspective view of a portion of a prior art aircraft nacelle 1520 comprising inlet lip 1521. Bulkhead 1528 and inlet lip 1521 define the nacelle interior chamber or D-duct 1530. Bulkhead 1528 also separates the D-duct 1530 from the interior portion 1531 of inner barrel 1512. An acoustic panel 1504 forms the interior portion of the inlet lip 1521. An IPS 1512 and its associated thermal insulation layer (not shown) are affixed upon the surface of the acoustic panel, and extend around inlet lip 1521, approximately to the highlight 1505. In the prior art system of FIG. 4, the inner barrel 1512, which is joined to the inlet lip 1521 by joint 1514, comprises one or more acoustic open cell panels 1506 for noise abatement. A second joint 1515 joins the nacelle inlet lip 1521 to the nacelle outer barrel 1516. The prior art system shown in FIG. 4 further comprises a parting strip 1507 at or near the highlight 1505 of the nacelle, depending upon the location of the stagnation point of the nacelle (i.e. the point on the nacelle inlet lip at which the free stream air impacts directly upon the nacelle inlet lip, and where the impacting air is stagnant). The IPS 1512 and parting strip 1507 are electrically connected (by means not shown) to power supplies of the type previously described.
Though such prior art nacelle inlet lips may be effective in attenuating engine noise and electro-thermally eliminating or minimizing ice buildup on engine nacelles, such prior art devices have at least some shortcomings. First, a heating element that is externally mounted on a nacelle inlet lip may be susceptible to damage from impacts by objects striking the inlet lip. In addition, externally applied heating elements may delaminate from the inlet lip outer skin during prolonged service. In addition, the adhesives used to bond porous, externally applied heating elements can at least partially block the acoustic openings in the heaters, thereby reducing the percentage of open area (“POA”) of the heaters, and decreasing the sound-attenuation capabilities of the inlet lip. Accordingly, there is a need for an acoustically treated nacelle inlet lip having integrally formed, embedded electro-thermal heating elements, and a sufficiently large POA to provide a substantial degree of engine noise-attenuation.